Traditionally, satellite systems use a single beam to cover a large geographic area. Within the coverage area, each carrier frequency is used only once. In 1995, the American Mobile Satellite Communications System became operational. This system uses a few spot beams to cover the continental United States, Alaska, and Hawaii. However, no two carrier frequencies are used simultaneously in the system. Since the available bandwidth limits the number of available channels, traditional satellite systems cannot support a large number of users.
In cellular communication systems, frequency reuse plans allow the same frequency to be used more than once within the system. Rather than use a single high power transmitter to cover a large geographic area, cellular systems employ a large number of low-power transmitters which broadcast a signal in relatively small geographic areas referred to as cells. Each cell may be only a few miles across, and theoretically could be as small as a few city blocks. By reducing the coverage area of the transmitter and creating a large number of cells, it is possible to reuse the same frequency in different cells. Thus, a single frequency may be used multiple times throughout the entire cellular system to increase caller capacity. For example, assume that a particular geographic region is served by a single high-powered transmitter having ten frequency channels. The system would be able to handle only ten simultaneous calls. The eleventh caller would be blocked because no other channels are available. If the same geographic region is divided into 100 cells and the same frequencies could be used in all cells, then 1,000 simultaneous calls could be supported. This cellular approach can be used in satellite systems to increase system capacity.
Unfortunately, immediate reuse of all available frequencies in adjacent cells is not practical because of co-channel interference. The actual boundaries of cells in the real world are ill-defined and subject to constant changes due to signal fluctuations. Thus, the coverage area in adjacent cells overlap and intermingle. A vehicle operating near the boundary of a cell would be in an ambiguous zone where the signal strength from two adjacent cells using the same frequency is roughly equal. This balanced zone or interference zone makes communications difficult. The mobile unit would lock first onto one transmission, then the other, as the signal strength from the transmitters in adjacent cells fluctuates. This constant hopping between transmissions would make communication impossible.
To avoid the problem of co-channel interference, cells operating on the same frequency are spatially separated so that the mobile unit operating within a cell receives the desired signal at a higher level than any potential interfering signal from co-channel cells. Cells operating at different frequencies are placed between any two co-channel cells. Thus, the mobile unit will change frequencies during hand-off as it approaches a cell boundary before entering the interference zone between any two co-channel cells.
In general, the power of any interfering signal diminishes with increasing distance between interfering users. A carrier frequency can be reused if the interference level is reduced sufficiently by separation between the co-channel calls. The interference level is measured by the carrier power to interference power ratio, C/l. The C/l ratio is the primary criteria used in designing frequency reuse plans.
From the foregoing, it should be apparent that the number of times a given frequency can be reused in a system is related to the separation distance or reuse distance between any two co-channel cells. Developing new frequency allocation plans which reduce the co-channel interference allowing greater reuse of frequencies without sacrificing signal quality would result in greater system capacity.